Nigerian President-elect Muhammadu Buhari's promised campaign to defeat Boko Haram could drive more militants over the country's borders, raising the need for cooperation between governments across the region, senior U.N. officials said on Thursday.

Speaking on the eve of the former army general's inauguration, they voiced hope that the new Abuja government would crush the Islamist militants accused of using women and children as sexual slaves and suicide bombers.

"There is this concern that success inside northeast Nigeria spells trouble for Niger, Cameroon, and even potentially Chad. So there is a lot of focus on regional cooperation," Robert Piper, U.N. assistant secretary-general and regional humanitarian coordinator for the Sahel, told a news briefing.

Thousands have been killed in Boko Haram's six-year-old insurgency in Africa's biggest oil producer. The group has already launched attacks on the territory of Niger, Cameroon and Chad.

"The promise from the elected president as a former chief of the army to defeat Boko Haram is also good news for us. However, our expectation is also that while the army is leading the battle and war against Boko Haram, international humanitarian law and human rights will be respected," said Munir Safieldin, U.N. deputy humanitarian coordinator for Nigeria.

Watching carefully

Boko Haram, whose name is roughly translated as "Western education is sinful," has been pushed from most of the territory it controlled in the past few months by Nigeria's army with assistance, at times, from soldiers provided by neighboring Chad, Niger and Cameroon.

Piper said that Lake Chad is "one of the natural places where Boko Haram will seek refuge if they can", adding: "We are watching northern Cameroon very carefully, we are watching southern Niger very carefully."

Nigeria's armed forces killed scores of Boko Haram militants and rescued 20 women and children as part of an operation in the Sambisa forest, the group's remaining hideout in the northeast, the military said on Saturday.

The army has freed some 700 women held by Boko Haram in recent weeks but the 200 schoolgirls abducted from the town of Chibok last year have not been among them.

"The women and girls who have been rescued from Boko Haram definitely now face a very difficult reality and many dilemmas," said Safieldin.

"Some of these girls have been reported to be pregnant, and definitely these are unwanted pregnancies as a result of rape, and some of them have been reported to have tested HIV positive," he said, noting that U.N. agencies are providing trauma counseling.